In electrical discharge machining, electric energy is applied in the form of discrete electrical pulses across the machining gap filled with a machining fluid or liquid dielectric (e.g., kerosine, transformer oil, distilled water) to effect a succession of electrical discharges between the tool electrode and the workpiece to remove material from the latter. As material removal proceeds, the tool electrode is advanced relatively toward the workpiece by servo feed means adapted to maintain the machining gap spacing substantially constant thereby to allow material-removal discharges to be successively created. The contamination of the machining gap region with chips, tar and gases produced by machining discharges may be clarified by continuously or intermittently flushing the fresh machining fluid into the gap and/or intermittently or cyclically retracting the tool electrode away from the workpiece to allow the fresh machining medium to be pumped into the machining gap and the machining contaminants to be carried away from the latter.
Parameters of individual and successive electrical discharges, i.e. pulse on-time .tau.on, peak current Ip and off-time .tau.off, are determinative of machining results, e.g. removal rate, surface roughness and relative electrode wear, and therefore are individually and in combination particularly adjusted to establish a particular machining condition suitable to achieve desired machining results.
As pointed out in U.S. Pat. No. 3,536,881 issued Oct. 27, 1970 to Kiyoshi Inoue, one of the problems arising in electrical discharge machining is the problem of changing current density as the working face of the tool electrode is juxtaposed with larger or smaller surface areas of the workpiece whereby discharges must occur over changing surface areas. Thus, a given total current resulting from uniform application of successive discharges will provide diminished current density as the working area increases and vica versa. With the machining gap maintained constant, for example, changes in the working surface area vary the current density with reduced performance even when various measures such as gap short-circuit and arc prevention are made in an attempt to hold the machining gap under an optimum machining condition. These particular deficiencies and inconveniences are overcome, as disclosed in the above-quoted patent, by maintaining the current density substantially constant by providing a current-control circuit responsive to the rate of the tool electrode with a gap-controlling servo for compensatorily varying the current. This is based upon the fact that, when the position of the tool electrode is adjusted by a servo-mechanism or servo feed means to maintain a substantially constant optimum machining gap, the rate of change of the feed with time is a function of the change in surface area exposed to electrical discharges along the machining surface. Consequently, a reduced surface area is equivalent to a higher removal rate and a correspondingly higher electrode feed rate, while an increased machining area is equivalent to a lower rate of material removal and a corresponding decrease in the rate of advance of the electrode.
Consequently, the aforesaid patent makes provision of sensing means responsive to the rate of advance of the electrode for indicating the variation in surface area exposed to the machining operation and mean-current control means in the power supply circuit electronically regulated in any of various manners by the rate sensing means. Specifically, as described therein, the mean-current control means may include a source of direct-current which is switched on and off to provide a train of discharge pulses whose parameters are modified to maintain the current density substantially constant, in response to change in machining area, by: (a) varying the on-time .tau.on of the pulses of the pulse train while maintaining the pulse frequency f substantially constant; (b) varying the off-time .tau.off of the train while maintaining the pulse frequency f substantially constant; (c) varying the on-time .tau.on of the pulses of the train while maintaining the off-time .tau.off of the pulses constant; or (d) varying the off-time .tau.off of the pulses of the train while maintaining the on-time .tau.on of the pulses constant.
The modification of individual pulse parameters has, however, been found to be disadvantageous in that preset machining conditions such as relative electrode wear are adversely affected and the machining operation becomes unstable. Hence, the earlier measures enunciated in the aforementioned U.S. Patent are of limited value in putting the concept into practice and are practically inconvenient.